Three neonates, two with unilateral renal vein thrombosis and one with unilateral dysplastic kidney, developed type 4 renal tubular acidosis, manifested by nonazotemic hyperkalemic metabolic acidosis with alkaline urine pH and reduced potassium excretion. Normal plasma concentrations of sodium, aldosterone, and renin activity, together with normal renal fractional excretion of sodium, supported the diagnosis of renal tubular acidosis type 4, subtype 5. Arginine HCl loading studies showed that despite their ability to bring the urine pH to less than 5.8, net acid excretion was inadequate relative to the corresponding plasma bicarbonate concentration. Treatment with oral bicarbonate resulted in sustained normalization of blood acid-base status and accelerated linear growth in the first two infants, in whom spontaneous recovery occurred by ages 8 and 15 months, respectively. At that time, the affected kidneys were extremely small with distorted collecting systems; the contralateral kidneys showed compensatory hypertrophy. In the third infant, persistent acidosis and growth failure resulted from medical noncompliance; the removal of the dysplastic kidney at 7 months of age was followed by the return to normal blood acid-base status and normalized tubular hydrogen and potassium excretion. We conclude that neonatal unilateral kidney disease can result in renal tubular subtype 5. Spontaneous recovery can be expected, presumably because of " autonephrectomy " of the affected kidney plus the compensatory hypertrophy of the contralateral kidney.